The art of the present invention relates generally to providing a thermally responding osmolyte draw solution for a forward osmosis assisted freshwater generation process wherein the osmolyte draw solution is thermally shifted between existing as a lower and a higher osmotic pressure osmolyte. The effect parlays both an advantageous efficiency toward freshwater recovery as well as providing a means for freshwater recovery from otherwise intractable very high total dissolved solids (TDS) feed waters. Embodiments of the present invention apply particularly well to increasing the recovery of freshwater from reverse osmosis processes, although the present invention is amenable for use with other freshwater extraction processes.
U.S. Pat. No. 3,721,621 presents a method wherein a forward osmosis process dewaters a feed water, such as seawater, employing a pH sensitive, high TDS, high osmotic pressure draw solution. A higher volume, dilute draw solution results. The pH of this dilute draw solution is thereafter shifted facilitating precipitation of solute, resulting in a lower TDS product than the feed water serviced. This prior art suffers from the consumable expense of the pH sensitive solute as well as pH shifting chemicals. Further the final product effluent remains relatively high in TDS, reducing its applicable value.
The prior art has attempted achieving increased recovery from reverse osmosis processes by improvements of commercial membrane rejection ratios as well as employing higher pressure amenable membranes and associated strengthened support structures. The burden is structural limitations due to the high osmotic pressures associated with increased TDS and recovery ratio. The present invention, in particular, resolves the high pressure limitations and performance frailties associated with the prior art. When combined with reverse osmosis processes as well as other freshwater extraction processes, the present invention affords a novel means to increase the freshwater recovery factor and concentrate TDS with the employ of low grade thermal energy.